Seatbelt pretensioners are known as one of the safety apparatuses for protecting occupants from impact in the event of collision of an automobile. Such seatbelt pretensioners are equipped with a micro gas generator which is activated by high temperature, high pressure gas generated from the gas generator in the collision, and take up the seatbelt slightly to protect the occupant. Application of such micro gas generator used for seatbelt pretensioners have been extended recently and for example, the generator is becoming widely used for activation of devices which reduces damage to the human body by suppressing forward movement of the occupant by instantly bouncing up the front end of the seat slightly in the collision, or which reduces damage to pedestrians by instantly bouncing up the hood slightly to create a gap between the hood and the engine in the collision with the pedestrian.
The micro gas generator used for such purposes comprises an aluminum cup body for filling a gas generating agent therein, a squib which ignites the gas generating agent and a holder for fixedly sealing the squib in the cup body, and is also called a micro gas generator.
For the gas generating agent used in the micro gas generator, an agent which generates a large amount of gas and has an ignition temperature lower than the brittle temperature of aluminum which is the material of the cup body has been used.
With increased awareness of environment and health in recent years, however, a gas generating agent which generates a large amount of gas and exhausts cleaner gas has been demanded. Such gas generating agent has a high ignition temperature of not lower than 300° C. (see U.S. Pat. No. 6,136,114).
When a micro gas generator using a gas generating agent having such a high ignition temperature is exposed to high temperature by fire on vehicles, embrittlement of the safety system and destruction by activation of the micro gas generator are expected to be caused.
On the other hand, for airbag inflators which are typical gas generators to cope with high temperature caused by fire on vehicles, one in which the strength of the housing is increased by thickening the board of the housing to prevent housing from exploding, one in which an agent which ignites automatically at a relatively low temperature (hereinafter referred to as “automatic igniting agent”) is put at a suitable position in the housing to make the gas generating agent burn under the temperature condition where the housing can stand the high pressure of the combustion gas (see Japanese Patent Application Laying Open (KOKAI) No. 2001-225711) and one in which an automatic igniting agent is formed into a film and placed so as to contact with both the inner surface of the housing and the gas generating agent (see Japanese Patent Application Laying Open (KOKAI) No. 9-328052) have been known.
These methods, however, have a defect of complicated structure and increased cost because of some structural installation inside the gas generator. In particular, since micro gas generators used for seatbelt pretensioners have a smaller amount of gas generating agent than airbag inflators, the amount of the automatic igniting agent to carry out the automatic igniting function is relatively increased, possibly affecting the combustion property.
An object of the present invention is to improve such defect and to develop a micro gas generator which has an automatic ignition function and activates before embrittlement of the cup body by heat even in a small amount without loss of the cleanliness of exhaust gas and the combustion property of the gas generating agent. The present inventors have conducted intensive studies to solve the above problems and have found that a gas generator which surely ignites automatically even with a reduced amount of automatic igniting agent can be obtained by forming an automatic igniting agent layer on the inner surface of a combustion chamber for the gas generating agent.